Carbon-based materials are used for negative active materials, and in particular, crystalline graphite with high crystallinity is generally used. This graphite has a layered structure, and lithium ions are intercalated from the edge of the layered graphite to the intervals of graphite layers during charging of a secondary battery, thereby producing a Li-graphite intercalation compound.
When graphite is used as a negative active material to prepare a negative electrode, the planes of the graphite layers are parallel to the plane of the collector, since most graphite is flake-shaped. Therefore, the edges of the graphite layers are aligned in a direction perpendicular to the positive electrode, and therefore lithium ions which are deintercalated from the positive electrode cannot easily intercalate to the graphite layers during charging.
In particular, when a, battery is charged at a high rate, lithium ions are insufficiently intercalated to the graphite layers, and discharge characteristics consequently deteriorate.
In addition, since a lithium secondary battery is generally charged under constant current and constant voltage (CC-CV) and it is discharged under constant current, lithium ions which are deeply intercalated to the crystalline graphite layers are not fully deintercalated when the battery is discharged at high rates, thereby deteriorating cycle life characteristics.
Cycle life characteristics of a conventional lithium secondary battery further deteriorate because the lithium ions that deintercalate are insufficient to intercalate to the graphite layers, and too many lithium ions remain in the graphite.
Electrical resistivity of a graphite-containing composition in the inner plane direction of a graphite layer (an (ab) plane or a (002) plane) is about 1000 times than that of in the plane direction of the graphite layer. Therefore, if the alignment of graphite can be controlled, anisotropy of graphite may decrease or it may be eliminated, and the graphite can be used in electronic appliances as well as in batteries.